The present invention concerns a fluorinated carbon, a method for its preparation, and its utilization as active electrode material.
Fluorinated carbons, which comprise the fluorides of carbon and the carbon-fluorine intercalation compounds, are known for their applications as lubricants on the one hand, and as cathode materials in lithium electrochemical generators on the other hand. The first application is based on the low surface energies stemming from the presence of hydrophobic Cxe2x80x94F groups. The second application is permitted by the relative facility of intercalation of lithium ions during the process of discharge of the generator, as well as by the marked reducing character of lithium as regards the Cxe2x80x94F bond.
Various methods are known for the preparation of fluorinated carbons, and give different products.
Carbon fluorides have been obtained by direct fluorination. A process of direct fluorination of carbon was described in 1934 [O. Ruff et al., Z. Anorg. Allgem. Chem., 217, 1(1934)]. It enabled a gray compound to be obtained, of composition CF0.92. A process of direct fluorination of graphite at temperatures of 410xc2x0 C. to 550xc2x0 C. enables a series of fluorinated carbons to be obtained, the composition of which was from CF0.676 to CF0.988 [W. Rxc3xcdorff et al., Z. Anorg. Allgem. Chem., 253, 281(1947)]. The carbon fluorides thus obtained correspond to a structure (CF)n in which the layers of carbon are constituted by an infinite network of hexagonal xe2x80x9cchair formxe2x80x9d cycles bonded together by an sp3 type of bond. The Cxe2x80x94F bond is purely covalent and the compounds are electrical insulators. Another method by direct fluorination, permitting a carbon fluoride having the formula (C2F)n to be obtained, and in which the Cxe2x80x94F bond is a covalent bond, was described by Y. Kita et al., J. Am. Chem. Soc., 101,3832 (1979).
The compounds thus obtained by direct fluorination, (CF)n in particular, are utilized at present as cathode material in commercial lithium batteries. These batteries discharge around 2.4 V to 2.5 V at a current density of about 1 mA/cm2.
The essential characteristic of graphitic fluorides is the high energy of the Cxe2x80x94F bond, which can be measured by, for example, ESCA (XPS) spectrometry and which gives values of the most intense peaks of the line F1sxe2x89xa7688.5 eV and the line C1sxe2x89xa7290 eV. Furthermore, the sp3 hybridization of carbon brings about an increase of the length of the Cxe2x80x94C bond within the hexagon. The parameter a of the structure, which is 2.46 xc3x85 in graphite, exceeds 2.50 xc3x85 in (C2F)n and (CF)n [N. Watanabe et al., xe2x80x9cGraphite Fluoridesxe2x80x9d, Elsevier (1988), p. 50]. Moreover, in all the cases of direct fluorination of a natural or synthetic graphite or of a coke, the temperature of fluorination is necessarily greater than 400xc2x0 C. if it is desired to obtain a compound which is rich in fluorine (F/C greater than 0.6) which can be used in a battery.
Carbon-fluorine intercalation compounds, in which F/C less than 0.5, have been obtained by various methods of fluorination of carbon at ambient temperature. A first method consists of reacting graphite with a gaseous mixture of F2+HF, and permits stage 1 intercalation compounds to be obtained with a composition going from C4F to C3.6F [W. Rxc3xcdorff et al., Chem. Ber., 80, 417 (1947)]. The stage s denotes the number of layers of carbon separating two successive layers of fluorine. Thus a compound of stage 1 has a sequence of lamination of the layers as C/F/C/F . . . , and a compound of stage 2 has the sequence F/C/C/F/C/C . . . .
Another known method consists of causing graphite to react with fluorine in the presence of HF or of a metallic fluoride such as LiF, SbF5, WF6, CuF2 or AgF, and enables intercalation compounds CxF of stages 1 to 4 to be obtained, with 2xe2x89xa6xxe2x89xa616 [T. Nakajima, et al., Z. Naturforsch. 36b, 1419 (1981)]. Likewise, the synthesis is known of similar products of stage 1 to stage 4 having a composition of C2F to C10F, by reaction at ambient temperature of graphite with an atmosphere of F2 containing a small quantity of a fluoride HF, AsF5, IF5 or OsF6. In all the intercalation compounds thus obtained, the F/C ratio is at most equal to 0.5. Now it is known that the capacity of a generator containing a fluorinated carbon as active material of an electrode increases with the proportion of fluorine. A proportion of fluorine lower than 0.5 is thus insufficient. Moreover, the fluorine contained in these compounds is less strongly bound to carbon, permitting it to have a mobility between the planes. Because of this, the fluorine can disintercalate, become dissolved in the electrolyte, and react with the lithium electrode, bringing about a phenomenon of self-discharge. These intercalation compounds prepared in the presence of HF or of a metal fluoride have an ionic character when the fluorine content is very low (F/C less than 0.1), or an iono-covalent character for higher fluorine contents (0.2 less than F/C less than 0.5). In any case, the bonding energy measured by ESCA gives a value less than 687 eV for the most important peak of the F1s line and a value less than 285 eV for that of the C1S line [T. Nakajima, Fluorine-carbon and Fluoride-carbon, Chemistry, Physics and Applications, Marcel Dekker (1995) p.13]. Moreover, the carbon remains hybridized in sp2 and the crystallographic parameter a in the plane remains in the neighborhood of 2.46 xc3x85 as in the case of graphite.
Carbon-fluorine intercalation compounds in which F/C greater than 0.5 have been obtained by a method of fluorination by means of a gaseous mixture of HF, F2, and a metallic or non-metallic fluoride MFn [A. Hamwi et al., Synt. Metals, 26, 89 (1988]. This method enables compounds of formula CFxMy to be obtained, of stage 1, having a F/C ratio comprised between 0.52 and 0.8 and a M/C ratio comprised between 0.02 and 0.06. This method has a disadvantage, however, for the preparation of fluorinated carbons intended to be used as active electrode material. The compounds obtained have relatively little stability when used as active material of an electrode in an electrochemical generator having a lithium negative electrode. This instability becomes evident as a loss of capacity of the battery by self-discharge, particularly at higher temperatures. This loss is principally due to the high content of the impurity M. Moreover, the compounds in which the value of y is very low, near to the lower limit y=0.02, are difficult to obtain by this method.
The present invention has as its object to provide a new fluorinated carbon having a lower proportion of impurities and thus a good stability when used in an electrochemical generator having a lithium negative electrode, as well as a sufficient fluorine content for its use in an electrochemical generator.
The invention thus has as its object a fluorinated carbon, a method for its preparation, and its utilization as active material of the positive electrode in a battery whose negative electrode is a lithium electrode.
The fluorinated carbon of the present invention is characterized in that:
it corresponds to the formula CFxMy in which x greater than 0.6, y less than 0.018, and M represents an element chosen from among I, Cl, Br, Re, W, Mo, Nb, Ta, B, Ti, P, As, Sb, S, Se, Te, Pt, Ir and Os;
its crystallographic parameter a, corresponding to the line (100) of the lattice, is such that 2.46 xc3x85xe2x89xa6axe2x89xa62.49 xc3x85;
the Cxe2x80x94F bond energy is characterized by the strongest lines F1s, and C1s at positions such that 687.5 eVxe2x89xa6F1sxe2x89xa6688.5 eV and 287 eVxe2x89xa6C1sxe2x89xa6290 eV, in the ESCA spectra.
Such a bond energy corresponds to an iono-covalent type of bond.
The method for preparing a fluorinated carbon of the invention is characterized in that:
during a first step, a carbon compound chosen from natural or synthetic graphites and graphitizable carbons with a mosaic texture by a thermal treatment are reacted with a gaseous mixture (HF+F2), in the presence of a fluoride MFn at a temperature between 15xc2x0 C. and 80xc2x0 C., where M represents an element chosen from among I, Cl, Br, Re, W, Mo, Nb, Ta, B, Ti, P, As, Sb, S, Se, Te, Pt, Ir and Os, and n represents the valence of the element M, with nxe2x89xa67;
during a second step, the compound obtained at the end of the first step is reacted with fluorine for 1-20 hours at a temperature between 20xc2x0 C. and 400xc2x0 C.
For the first step, there is preferably used a natural or synthetic graphite or a graphitizable carbon with a mosaic texture, having a particle size less than 100 xcexcm, preferably between 4 and 30 xcexcm. Among the graphitizable carbons, coal tar cokes and petroleum cokes are preferred.
The fluoride MFn is preferably chosen from among IF7, IF5, BrF5, ClF3, ReF6, WF6, MoF6, TiF4, NbF5, T2F5, PF5, AsF5, SbF5, BF3, SF6, SeF6, IrF6, OsF6, TeF6, and PtF6. IF7 and IF5 are particularly preferred.
The gaseous mixture used during the first step of the method is preferably constituted by 0.8-1.2 moles of MFn, 2-6 moles of HF, and 6-8 moles of F2 for the treatment of 8-14 moles of C.
During the second step of the method, the temperature is preferably comprised between 20xc2x0 C. and 200xc2x0 C. when the starting compound is based on graphitizable coke, and between 80xc2x0 C. and 400xc2x0 C. when the starting compound is based on graphite. Moreover, it is preferable to operate under a partial pressure of F2 comprised between 5xc3x97104 and 105 Pa.
The first step of the method of the invention can be carried out either by introducing the compound MFn into the reactor or by preparing it in situ by direct action of fluorine on the element M or on one of its oxides MOq for which 2q less than nxe2x89xa67, or on one of its lower fluorides MFp for which p less than nxe2x89xa67.
The second step of the method can be carried out immediately after the first on the product as obtained. It can likewise be carried out after having washed the product obtained in the first step, for example with pure water or acidified water, or with an organic solvent chosen from among alcohols, ethers, esters or carboxylic acids, preferably having at most 8 carbon atoms.
The fluorinated carbons of the present invention are particularly useful as electrode materials in an electrochemical generator having a lithium negative electrode.
An electrochemical generator according to the invention comprises a lithium negative electrode and a positive electrode, with a separator placed between them, and an electrolyte. It is characterized in that the positive electrode contains, as active material, a fluorinated carbon:
which corresponds to the formula CFxMy in which x greater than 0.6, y less than 0.018, and M represents an element chosen from among I, Cl, Br, Re, W, Mo, Nb, Ta, B, Ti, P, As, Sb, S, Se, Te, Pt, Ir and Os;
of which the crystallographic parameter a, corresponding to the line (100) of the lattice, is such that 2.46 xc3x85xe2x89xa6axe2x89xa62.49 xc3x85;
in which the Cxe2x80x94F bond energy is characterized by the strongest lines F1s and C1s at positions such that 687.5 eVxe2x89xa6F1sxe2x89xa6688.5 eV and 287 eVxe2x89xa6C1sxe2x89xa6290 eV, in the ESCA spectra.
In an electrochemical generator according to the invention, the positive electrode is constituted by a composite material comprising fluorinated carbon according to the invention, a compound assuring electronic conduction, and possibly a binder.
The material assuring electronic conduction can be chosen from among carbon blacks, acetylene black, powdered graphite, cokes, and carbon fibers.
As binder there can be used a poly(ethylene oxide), a PVDF (polyvinylidene fluoride), a EDPM (ethylene-propylene-diene monomer), a poly(acrylonitrile), or a styrene-butadiene rubber (SBR). The copolymerized PVDF sold by the company Elf Atochem under the name Kynar Flex is particularly preferred.
The negative electrode can be constituted by a foil or a film of lithium or of a metallic alloy of lithium (LiAl, for example), or of carbon-lithium (LixC6, for example).
The electrolyte is a liquid electrolyte constituted by a salt in solution in a polar solvent. The salt can be chosen from among the compounds LiAsF6, LiBF4, LiCF3SO3, LiClO4, LiN(CF3SO2)2, LiPF6. The solvent can be chosen from among diethyl carbonate, diethoxyethane, dimethoxymethane, dimethyl carbonate, propylene carbonate, ethylene carbonate, or xcex3-butyrolactone.
The separator can be based on a non-woven polyethylene and/or polypropylene. By way of example, a microporous film of polyethylene can be used, particularly a film sold by the company Hoechst Celanese under the name of Celgard 2400 or 2502.